错误RuntimeWarning: Glyph xxxxx missing from current font的产生原因解析,检测当前字体是否包含某字符

RuntimeWarning: Glyph xxxx missing from current fontmatplotlib的经典错误。
原因大家都清楚:字体不匹配,显示不了对应的字符。
现象就是:本该显示的字符,显示为方框了,一般出现在汉字当中。

那这个错误是怎么产生的呢?

调试信息

 d:\ProgramData\Anaconda3\lib\site-packages\matplotlib\backends\backend_agg.py:201:
  RuntimeWarning: Glyph 39064 missing from current font.
  font.set_text(s, 0, flags=flags)

根据traceback信息可知:

  1. 提示当前字体中找不到Glyph 39064
  2. 错误发生在backend_agg.py
  3. 出错代码在font.set_text(s, 0, flags=flags)

问题分析

  1. Glyph是象形文字的意思,可以理解为符号,那39064是什么呢?通过查找汉字unicode编码表可知,39064对应的汉字就是。从字面意思上看,错误信息就是表示在当前设置的字体中找不到对应字符不单汉字会出现这样的错误提示,只要当前字体中不存在对应字符都会报这个错误
  2. backend_agg.pymatplotlib的绘图后端模块。
  3. font.set_text(s, 0, flags=flags)属于RendererAgg类中的draw_text方法。相关代码为:
        font = self._get_agg_font(prop)
        if font is None:
            return None
        # We pass '0' for angle here, since it will be rotated (in raster
        # space) in the following call to draw_text_image).
        font.set_text(s, 0, flags=flags)

再次追踪 _get_agg_font(prop)。

def _get_agg_font(self, prop):
    """
    Get the font for text instance t, caching for efficiency
    """
    fname = findfont(prop)
    font = get_font(fname)
    font.clear()
    size = prop.get_size_in_points()
    font.set_size(size, self.dpi)
    return font

font = get_font(fname)
from matplotlib.font_manager import findfont, get_font
问题又回到了font_manager模块了!

_get_font = lru_cache(64)(ft2font.FT2Font)
# FT2Font objects cannot be used across fork()s because they reference the same
# FT_Library object.  While invalidating *all* existing FT2Fonts after a fork
# would be too complicated to be worth it, the main way FT2Fonts get reused is
# via the cache of _get_font, which we can empty upon forking (in Py3.7+).
if hasattr(os, "register_at_fork"):
    os.register_at_fork(after_in_child=_get_font.cache_clear)


def get_font(filename, hinting_factor=None):
    # Resolving the path avoids embedding the font twice in pdf/ps output if a
    # single font is selected using two different relative paths.
    filename = os.path.realpath(filename)
    if hinting_factor is None:
        hinting_factor = rcParams['text.hinting_factor']
    return _get_font(os.fspath(filename), hinting_factor,
                     _kerning_factor=rcParams['text.kerning_factor'])

from matplotlib import afm, cbook, ft2font, rcParams

ft2fontmatplotlib的模块,在我的计算机上文件名是ft2font.cp37-win_amd64.pyd,看来是用其他语言编译的模块。
通过查找可知,ft2font模块的源码即https://github.com/matplotlib/matplotlib/blob/master/src/ft2font.cpp

static FT_UInt ft_get_char_index_or_warn(FT_Face face, FT_ULong charcode)
{
     
    FT_UInt glyph_index = FT_Get_Char_Index(face, charcode);
    if (!glyph_index) {
     
        PyErr_WarnFormat(NULL, 1, "Glyph %lu missing from current font.", charcode);
        // Apparently PyErr_WarnFormat returns 0 even if the exception propagates
        // due to running with -Werror, so check the error flag directly instead.
        if (PyErr_Occurred()) {
     
            throw py::exception();
        }
    }
    return glyph_index;
}

由此可知,这条错误从哪儿来了的!就是字体里没有对应的字符

检测当前字体是否含有该字符

根据fonttools写了一个简易检测当前字体是否含有该字符的小工具。

import sys
from itertools import chain

from fontTools.ttLib import TTFont
from fontTools.unicode import Unicode

font_path=r"c:\windows\fonts\simhei.ttf"
def font_validation(input_string,font_path):
    font =TTFont(font_path)
    chars = chain.from_iterable([y + (Unicode[y[0]],) for y in x.cmap.items()] for x in font["cmap"].tables)
    for i in input_string:
        char=ord(i) 
        # 输出字符、10进制Unicode编号、16进制Unicode编号、Unicode名称、是否包含在字体内   
        print(i,char,hex(char),Unicode[char],char in (x[0] for x in chars))
    font.close()

font_validation("标题₩",font_path)

从结果看,韩元符号黑体是没有的。

26631 0x6807 CJK UNIFIED IDEOGRAPH-6807 True39064 0x9898 CJK UNIFIED IDEOGRAPH-9898 True65510 0xffe6 FULLWIDTH WON SIGN False

使用黑体绘图测试下:
在这里插入图片描述

import matplotlib.pyplot as plt
from matplotlib.font_manager import FontProperties

font = FontProperties(fname=r"c:\windows\fonts\simhei.ttf", size=30) 
plt.title("标题₩", fontproperties=font)
plt.show()

使用gulim绘图测试下:
在这里插入图片描述

import matplotlib.pyplot as plt
from matplotlib.font_manager import FontProperties

font_path=r"C:\Users\Administrator\AppData\Local\Microsoft\Windows\Fonts\gulim.ttc"
font = FontProperties(fname=font_path, size=30) 
plt.title("标题₩", fontproperties=font)
plt.show()

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